Bistable magnetic valves are for example used in rapid-switching hydraulic actors and hydraulically-assisted servo valves in motor vehicles. A magnetic valve used for the opening and closing a hydraulic passage can in this case be held in a relevant end position by magnetic remanence forces of a flow line part (yoke). To keep the forces needed to hold the valve down, such valves are usually pressure compensated so that the hydraulic pressure does not exercise any force in the sense of activation of the valve in this case (“pressure-compensated valve”).
FIG. 1 shows schematically the basic structure of a known bistable magnetic valve (Internet site at www.sturmanindustries.com, November 2002). The magnetic valve 1 has a housing 3, through which hydraulic channels 5, 7 and 9 run into the inside of the housing 3. In the inside of the housing 3 a magnetic armature 11, also called in this type of valve a valve needle or “spool” is guided so that it can move.
In FIG. 1 the magnet armature 11 is shown in its left-hand end position. In this position all hydraulic channels 5, 7 and 9 have a fluid interconnection (valve open) to each other through suitably arranged channels inside the magnetic armature 11 (not shown), whereas in a right-hand end position of the magnetic armature 11 these hydraulic channels 5, 7 and 9 are separated from each other (valve closed).
To switch the magnetic armature 11 between its two stable positions the magnetic valve 1 has two magnetic windings or magnetic coils 13 and 15 which are supplied with an electrical current pulse in order on the one hand to pull the magnetic armature 11 to the left to open the valve (magnetic coil 13) and on the other hand to close the valve 1 by pulling it to the right (magnetic coil 15). The magnetic flux induced by the magnetic coils 13 and 15 is guided and amplified in a known way through the flux guidance parts 17 and which, like the magnetic armature 11, are made of soft magnetic material.
Typical applications for these types of bistable magnetic valves are electro-hydraulic devices in motor vehicles, for example hydraulically-assisted diesel injection valves and hydraulically-activated valve drives (inlet/outlet valves) for internal combustion engines.
A circuit arrangement based on internal developments by the applicant for controlling a bistable magnetic valve, such as the magnetic valve 1 shown in FIG. 1, is shown in FIG. 2, whereby, to simplify the diagram, only the part of the circuit for supplying current to a magnetic coil 13 of a bistable magnetic valve in FIG. 2 is shown. In practice an identical circuit part is used to control the second magnetic coil.
FIG. 2 shows a power supply source for providing a supply voltage Uv, which in the exemplary embodiment shown is 48V and for example represents the output voltage of a DC/DC voltage converter of an on-board power supply in a motor vehicle.
The magnetic coil 13 is represented in FIG. 2 by its equivalent circuit (Inductor Lcoil and ohmic resistor Rcoil). Typical values are Lcoil=150 μH and Rcoil=1Ω. The current pulse used to switch over the magnetic valve (coil current) has a typical duration of 1 ms and a peak value of around 20 A. To achieve this peak value of the coil current a voltage of around 20V would be sufficient for the given value of Rcoil. However in order to build up the coil current rapidly at the beginning of the current pulse, a significantly higher voltage, namely the supply voltage Uv of 48V is fed to magnetic coil 13. When the supply voltage Uv is applied to magnetic coil 13 at the beginning of the switchover process, current I through the winding increases in accordance with the equation di/dt=Uv/Lcoil, from which it can be seen that the increase of the coil current I becomes steeper, the greater the voltage Uv delivered to the winding.
In practice the power dissipation produced in the coil resistor Rcoil limits the switching frequency of the magnetic valve. With high pressure diesel systems (“Common Rail”) with multiple injections for example, this limits the number of injection processes possible at high engine speeds. In a similar way the power dissipation produced in a valve controller using these types of magnetic valve also represents a technical challenge which often forces additional, expensive heat dissipation methods to be used. In order it to limit the power dissipation in the example shown, the coil current is adjusted for the duration of the current pulse, as described in detail below.
The circuit is activated by the “Enable” signal which can be produced for example by a microcontroller. If this enabling signal or activation signal Enable has a logical high level a transistor (FET) T2 is switched on to the gate of which the enable signal is applied. In addition, at least at the beginning of the current pulse, a transistor (FET) T1 is also switched on by also applying the high level to the gate of transistor T1 and by doing this indirectly via an AND element 30, which at an input connection IN1 receives the enable signal and which is connected at its output with the gate of T1. The transistors T1 and T2 form a current switching device which in a first switching state (T1 and T2 switched on) feed the supply voltage Uv to the magnetic coil 13. In this state the coil current flows from the supply voltage source via T1, Lcoil, Rcoil, T2 and finally via coil current measuring resistor Rsense back to the supply voltage source. In second circuit state T1 is however switched off by applying a low level to a second input connection IN2 of the AND element 30, so that an free-running circuit is provided for the coil current. The level at the second input connection IN2 is activated, as described in more detail below, depending on coil current. In this state the coil current flows from a pole (0) of the supply voltage source via a diode D2, the magnetic coil 13, the transistor T2 and finally the measuring resistor Rsense back to pole 0 of the power supply source.
The current switching device T1, T2 serves as a control element within the framework of the adjusting the coil current I for the duration of the current pulse to a desired target current (setpoint) of 20 A here. For this purpose an actual value of the coil current is measured as a voltage drop at measurement resistor Rsense, amplified by means of a differential amplifier Diff_Amp and the amplified voltage compared by means of a comparator Komp with a reference voltage Vref for which the output signal is presented to the second input IN2 of the AND element 30.
These components, which can be seen at the bottom of FIG. 2 thus form a setpoint specification unit for specifying a setpoint of the coil current, a measuring unit for measuring an actual value of the coil current as well as a regulator for forming the system deviation signal from the setpoint and the actual value of the coil current and for controlling the power switching device.
Comparator Komp has a hysteresis, so that if its output exceeds an upper limit Vref+Vhyst at low-level, it switches to low level and only switches back to high when it falls back below a lower limit Vref−Vhyst.
At the beginning of the current pulse the voltage drop at Rsense is small. The current now rises over time and the voltage drop amplified by the differential amplifier Diff_Amp exceeds the upper limit Vref+Vhyst. The output of comparator Komp switches to low, whereon the output of the AND element 30 also switches to low. This switches off the transistor T1. The corresponding timing graph of the coil current I can be seen from FIG. 3 (rising edge).
Driven by the counter EMF (Electromotive Force) of the inductor Lcoil the potential at the source of the transistor T1 will fall until (e.g. at around −0.7V) the free-running diode D1 becomes conductive and accepts the coil current. The coil current continues to circulate in the free-running circuit (D2, Lcoil, Rcoil, T2, Rsense) in which case its value falls slowly. If it has fallen far enough for the voltage drop amplified by the differential amplifier to exceed the lower limit Vref−Vhyst at the measuring resistor, the output of the comparator switches back to high, whereon, switched via the AND element 30, the transistor T1 switches back on. As a result the coil current will rise again until the switch-off point described above is reached once more. The current thus oscillates to and fro periodically between the lower and the upper limit, as can be seen from the center area of FIG. 3.
At the end of the current pulse the Enable signal now switches to low so that the transistors T1 and T2 are switched off at the same time. Driven by the counter EMF of the inductor Lcoil, the voltage at the magnetic coil 13 switches over and there is a current flow through the diodes D1 and D2 from the magnetic coil 13 into the power supply source. The inductor thus discharges (recirculates) into the power supply. In this phase the EMF collapses and the voltage at the magnetic coil 13 as well as the coil current drop rapidly to zero. This can be seen in the right-hand part of FIG. 3 (falling edge).
The power dissipation of this circuit arrangement is essentially determined by the switching times of transistor T1, in that the drain-source voltage for each switchover process passes through the potential area between 0 and 48V at full coil current. The instantaneous peak power dissipation at transistor T1 can in this case be around 1 kW for example. A reduction of the switching times for the purposes of reducing the power dissipation at T1 comes up against the problem however of the associated increase in EMC (electromagnetic compatibility), since as switching times become shorter the frequency spectrum created will increasingly contain higher frequency components. It is thus true to say that a compromise between power dissipation (heating up) and EMC disturbance created is to be found, in which case the room for maneuver is as a rule very small.